Reliable power supply and power quality are always the hot topics in industry. Unpredictable voltage drop or power shutdown usually interrupts the operating process or even damages equipment. Thus, many sensitive loads rely on UPS (Uninterruptible Power Supply) systems to maintain the stability of power supply lest the operating equipment be interrupted by a power failure suddenly.
Refer to FIG. 1 for a conventional line-interactive UPS system. Normally, the voltage at the utility power end 2 is transferred to a load 5 via a primary thyristor 3 and a load transformer 4. When detecting the voltage at the utility power end 2 abnormally (an instantaneous voltage drop or a sudden power interruption), the UPS system 1 is started up immediately. The power output by the UPS system 1 is sent to the load 5 via a secondary thyristor 6 lest the load 5 be shut down.
When the voltage of the utility power end 2 is interfered, the UPS system 1 has to shift the power of the load 5 within 1-5 ms lest any type of power interruption should occur. Within the 1-5 ms duration of load shifting, the distorted voltage waveform still applies to the load transformer 4 and causes the deviation of the flux linkage of the load transformer 4. When the UPS system 1 has completely taken over the voltage for the load and restored to the rated value, the flux linkage of the load transformer 4 may have exceeded the regulated operation range, which will cause a serious inrush current. Normally, the inrush current caused by magnetic saturation may reach as high as 2-6 times of the rated load current and last for several cycles of the utility power. The inrush current may cause the drop of voltage in the load circuit or even trigger the overcurrent protection mechanism of the UPS system. Once the overcurrent protection mechanism is triggered, the UPS system stops operating.
Many methods had been proposed to inhibit the inrush current caused by magnetic saturation of a transformer. Among them, directly controlling the output voltage of the UPS system is regarded as a simple and effective method. For example, in pp. 678-683 proceedings of 11th International Conference on Harmonics and Quality of Power, 2004, L. Ban and T. H. Ortmeyer proposed a paper “Improved Motor Starting Capability of Three Phase UPS Inverters”, wherein the output voltage of a UPS system is decreased by detecting value of the inrush current. In another method, the inrush current is inhibited via controlling the phase angle of the output voltage of the UPS system, wherein the voltage is output to the load transformer when the voltage waveform is at a phase angle of 90 degrees. For example, V. Zaltsman proposed a paper “Inrush current control for equipment powered by UPSs” in pp. 19.4/1-19.4/7 INTELEC'89 Conference Proceedings, 1989. However, in the abovementioned methods, the UPS system may be unlikely to instantly output the rated voltage required by the load, which exposes the load to a distorted voltage waveform for a longer duration, increases the probability of shutdown, or even damages the load. Besides, the abovementioned methods are unlikely to perform a fast load shifting to provide a stable power for the load when power fails or voltage drops dramatically.